1. Field of the Invention
The present invention relates to a vertical wafer boat which is used to support semiconductor wafers when the semiconductor wafers are put into a vertical heat treatment furnace such as a low pressure CVD (chemical vapor deposition) system.
2. Discussion of the Background
Semiconductor wafers are put into various kinds of heat treatment furnaces for treatment such as oxidation, diffusion and thin film deposition. A wafer boat has been used as a supporting tool to support numbers of semiconductor wafers for entry into and takeout from a heat treatment furnace. It is known that such a wafer boat is classified according to the structure of heat treatment furnaces into a horizontal boat for entry of semiconductor wafers into and takeout from a furnace through a lateral side of the furnace, and a vertical boat for entry of semiconductor wafers into and takeout from a furnace through an upper end or a lower end of the furnace.
Most of low pressure CVD systems which are used to form e.g. a silicon nitride film on a surface of semiconductor wafers are vertical. Numbers of semiconductor wafers which are inserted in and supported by a vertical wafer boat have been put from beneath into a reaction chamber kept at 400-850.degree. C. for treatment. This means that the wafer boat is subjected to repeated thermal stress due to a reciprocation between the reaction chamber at a high temperature and the outside at a low temperature.
Vertical heat treatment furnaces such as low pressure CVD systems have an inlet at a lower end thereof formed with a cooling unit such as a water jacket in some cases. A great difference in temperature between a lower portion and an upper portion of the wafer boat might be created to have fear of failure due to thermal stress. This necessitates use of a wafer boat which has a lower end provided with a pedestal having a height of not less than 5 cm for shielding heat-radiation with respect to a wafer boat for elongated vertical heat treatment furnaces.
As such a vertical wafer boat, e.g. the structure shown in FIG. 3 has been disclosed in JP-B-94105693. The wafer boat 10 in the publication includes a jig body 20 made of silicon carbide, a lower plate 30 made of silicon carbide coupled to a lower end of the jig body 20 by a suitable way such as bonding or engagement, and an upper plate 40 made of silicon carbide coupled to an upper end of the jig body 20 by bonding.
The jig body 20 of the wafer boat shown in FIG. 3 is formed in a substantially semicylindrical shape, and has an inner peripheral surface of a side wall thereof formed with a plurality of (e.g. four) ridges 21a-21d in a vertical direction to support semiconductor wafers to be heat-treated. The side wall has a suitable number of windows 22a-22c formed therein between the ridges 21a-21d in order to prevent the jig body 20 from being failure due to a thermal shock and to make the jig body 20 lighter and decrease heat capacity of the wafer boat for effective heat transfer. The ridges 21a-21d have a plurality of wafer supporting grooves 23a-23b formed thereon, respectively.
The lower plate 30 is constituted by a first plate 31 coupled to the lower end of the jig body 20, a second plate 32 for placement on a table of a heat treatment furnace, and a hollow cylindrical member 33 coupling the first plate 31 and the second plate 32. A wafer boat constituted only by the first plate is generally used as well.
In the wafer boat 10, slits 24, 31b and 33b are formed to extend in a lower portion of the jig body 20, the first plate 31 and the second plate 32 of the lower plate 30, and the hollow cylindrical member 33 to cut out these members at a single location (the slit formed in the second plate 32 is not shown). These slits prevents the lower plate 30 from being broken due to thermal stress.
When a separate pedestal is arranged at the lower end of the conventional vertical wafer boats, the contacting surfaces between the lower surface of the wafer boat and the pedestal have a deposited film by CVD thereon, creating a problem in that removal of the deposited film causes fine dust particles to contaminate semiconductor wafers.
Although a so-called pedestal equipped wafer boat which has a lower surface provided with a pedestal does not involve such a problem, such a wafer boat creates problems in that the presence of the pedestal makes the wafer boat heavier and cumbersome, increases heat capacity to enlarge a change in temperature in a furnace when the wafer boat is put into the furnace, and particulates are likely to be caused due to a stay of a reaction gas in the pedestal.